These deployments provide two complete copies of the data set at all
times in addition to the primary. These replica sets provide
additional fault tolerance and high availability. If the primary is unavailable, the replica
set elects a secondary to be primary and continues normal
operation. The old primary rejoins the set when available.

Since the arbiter does not hold a copy of the data, these deployments
provides only one complete copy of the data. Arbiters require fewer
resources, at the expense of more limited redundancy and fault
tolerance.

However, a deployment with a primary, secondary, and an arbiter
ensures that a replica set remains available if the primary or the
secondary is unavailable. If the primary is unavailable, the replica
set will elect the secondary to be primary.